AZ31鎂合金之磷酸鹽/過錳酸鹽化成皮膜微結構與成長機制探討

Abstract

本研究探討過錳酸根離子濃度與化成溫度對磷酸鹽/過錳酸鹽化成皮膜性質的影響，利用微結構的觀察以及業界常用皮膜性質評估方法做綜合的比較。實驗結果顯示化成溫度40℃下所得皮膜增厚速率較60℃快，皮膜裂紋較寬大；經SEM表面形貌觀察，發現皮膜結構較鬆散，附著性不佳，皮膜主要由氧、鎂、磷、錳組成。TEM觀察化成溫度40℃，添加10gl-1過錳酸鉀，皮膜結構由柱狀層及與鎂底材相鄰之多孔層組成；化成溫度60℃，添加10gl-1過錳酸鉀所得皮膜由柱狀層、非晶層和與鎂底材相鄰之多孔層三層結構組成。XRD分析，皮膜主要為非晶的結構，在皮膜表面無法解析出二氧化錳的訊號。 化成溫度低者，皮膜增厚速率快，皮膜附著性不佳，化成溫度較高，皮膜附著性較佳，且增加過錳酸鉀含量有利提升皮膜附著性。交流阻抗測試結果，化成溫度較高者，皮膜阻抗值較高，抗蝕性較佳，可見化成溫度較高者，有利於皮膜抗蝕性提升。鹽霧試驗發現皮膜阻抗值較高者，皮膜抗蝕性較佳，但鹽霧試驗時間較長後因皮膜厚度較薄，無法提供較長時間保護效果，腐蝕發生後在試片表面可偵測到強烈錳的訊號，此因二氧化錳抗蝕性較佳，腐蝕發生後氧及錳的訊號強度大幅上升。

This study details the effects of solution temperature and potassium permanganate (KMnO4) concentration on the properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy. The growth the coating was substantially suppressed as the solution temperature was increased from 40 to 60℃. Moreover, the coating formed at 40℃ exhibited a porous structure mainly composed of oxygen, magnesium, phosphorus and manganese species, and had poor adhesion to the substrate as characterized by SEM. Cross-sectional TEM further revealed that when immersed in the solution with 10 gl-1 KMnO4, the coating formed at 40℃ comprised an cellular overlay and a porous inner layer contacting with the substrate, while that formed at 60℃ exhibited a three-layered structure: a cellular overlay, an amorphous intermediate layer and a porous inner layer. Increasing solution temperature improved the coating adhesion to the substrate. Adding more KMnO4 to the solution resulted in the coating having higher impedance resistance and hence better corrosion protective properties after a short period of salt spray test. The corrosion resistance of the coating, however, degraded markedly during prolonged salt spray test. This is due to the thinner coating formed in the solution containing larger amounts of KMnO4. Finally, the coating after salt spray test contained more manganese species than the as-plated coating. Manganese oxide, which was likely present in the coating, seemed to enhance the corrosion resistance of phosphate/permanganate coating.